1. Field of the Invention
This invention relates to a variable magnification copying apparatus, more particularly, a copying apparatus capable of continuously or stepwisely changing the magnification. (Throughout the Specification and Claims, the term "magnification" is intended to cover enlargement, reduction and unit magnification.)
2. Description of the Prior Art
In electrophotographic machines, a photosensitive member (an image bearing member) is electrically charged, and then exposed, through optical elements, to image light which is reflected by the original to be copied.
In the electrophotographic machines capable of changing the magnification, the boundaries between the range of the photosensitive member within which the image of the original is formed and the range thereof outside such a range, is not constant, that is, it changes with the magnification selected. The area not exposed to the image light is not deprived of the electric charge and will be developed at the subsequent developing station. Further, the toner deposited to such an area is transferred to the transfer material at the transfer station to undesirably provide a black frame on the transfer material. This is disadvantageous also from the economical standpoint, since it wastes the developer agent, also since it raises the frequency of the replenishment of the developer agent. In addition, it puts much load on the subsequent cleaning station wherein the untransferred developer is removed from the photosensitive member. This problem has been solved by an additional exposure by which the area of the photosensitive member which is not exposed to the image light is exposed to additional light to remove the charge from such an area (hereinafter called "margin erasure").
However, in a variable magnification copying apparatus wherein the magnification can change continuously or stepwisely, the unilluminated area also changes continuously or stepwisely, so that the width of the additional exposure or the margin erasure has to be controlled continuously or stepwisely. A highly efficient device for charge removal for the margin erasure has been desired.